1. Field of the Invention
The present invention relates to a new and improved amplifier unit for driving an electrodynamic loudspeaker arranged in a cabinet or box along with a device which is coupled with the loudspeaker and which adds acoustic energy in a limited frequency range with the same polarity or unipolarly to that energy which is delivered from the front of the loudspeaker.
2. Discussion of the Background and Material Information
Belonging to the foregoing are, for example, bass reflex cabinets or boxes with amplifier units serving for driving or powering the loudspeakers or loudspeaker elements arranged in the bass reflex cabinet. Normally two resonance circuits are coupled with one another in a bass reflex cabinet. The one resonance circuit is constituted by the loudspeaker and the other resonance circuit by the aforementioned coupled device which, for example, comprises a port or opening in the cabinet which is coupled by the air contained in such cabinet with the rear or back of the diaphragm of the loudspeaker. Such bass reflex cabinets or boxes normally augment the acoustics within a narrowly defined frequency range. From the Journal of the Audio Engineering Society, Volume 19, No. 6, June 1971, the article authored by A. N. Thiele, entitled: "Loudspeakers in Vented Boxes: Part II", pages 192 to 204, it is known in this technology to drive such loudspeakers by an amplifier having negative output impedance, resulting in improved adaptation of the loudspeaker to its cabinet or box and vice versa.
The drawback of this combination of bass reflex cabinet or box and amplifier resides in dimensioning difficulties. These combinations have a frequency response in the bass range whose descent towards null is of the fourth order. Consequently, they possess a poorly controllable transient oscillation behavior and an unfavorable phase response.
A further such amplifier unit for driving the voice or moving coil of a bass loudspeaker is known, for instance, from the German Patent Publication No. 2,713,023 and the cognate U.S. Pat. No. 4,118,600, granted Oct. 3, 1978. Here, the amplifier unit has an output impedance which is equivalent to a negative resistance connected in series with a parallel resonance circuit. The negative resistance has practically the same value as the resistance of the voice coil. Due to the operation of the bass loudspeaker with such type amplifier there can be obtained a change of the bass loudspeaker which is equivalent to a change in the mechanical parameters of the loudspeaker, like, for example, its mass, compliance and damping. In other words, with these measures the resonance frequency of the loudspeaker is counteracted and at the same time there is produced a different resonance frequency which is better tuned to the cabinet and the device coupled with the rear or back of the diaphragm.
This prior art amplifier unit affords improvements in the frequency response of the therewith coupled loudspeaker, which is limited to the low-frequency range. Here, there is present a system of the fourth order possessing the previously noted drawbacks. No improvements can be realized for the mid- and high-tone ranges.
In European Patent No. 0,322,679, published Jul. 5, 1989, there is disclosed a loudspeaker system where the front side of a diaphragm drives a first resonator and the rear side of such diaphragm drives a second resonator, and thus, there is not generated any sound or acoustics which act directly upon the external surroundings. In contrast to a system containing a single resonator, it is thus possible to augment the frequency range where there arises resonance and thus bass intensification or amplification. By driving such system with an amplifier exhibiting negative output impedance there can be improved the frequency response for low frequencies.
The drawback of this system resides in the fact that it only provides improvements for low frequencies and there must be provided an additional loudspeaker which radiates tones of such frequencies which upwardly merge with the resonance frequencies of the higher-frequency resonator. Furthermore, there is here involved a system of at least the fourth order which is prone to the aforementioned disadvantages.